It has become increasingly economical to employ enhanced oil recovery techniques for recovering heavy crude oil. One particularly advantageous enhanced recovery method is the injection of high pressure gaseous carbon dioxide into an oil well. This procedure takes advantage of the high solubility of carbon dioxide in crude oil, together with the fact that the viscosity of the crude oil-carbon dioxide solution is significantly lower than the crude oil alone. Consequently, even heavy crude oils can be recovered by injecting gaseous carbon dioxide into a subterranean formation in an amount sufficient to saturate the contained oil, followed by the withdrawal of the low viscosity crude oil-carbon dioxide solution from the formation.
One consequence of this particular enhanced recovery technique, however, is that the gaseous phase recovered at the wellhead is contaminated with carbon dioxide. Since this carbon dioxide disadvantageously reduces the value of the recovered natural gas mixture, it is normally removed prior to further use of the natural gas. At present, a variety of separation techniques are available for this purpose.
A process currently practiced for removing carbon dioxide from gas streams is by physical or chemical washing or absorption. Solvents commonly used for these procedures include methanol, amines, propylene carbonate, potassium carbonate and N-methyl pyrolidone. With the absorption approach, operating expenses are strongly influenced by the concentration of carbon dioxide in the gas stream to be treated. As the carbon dioxide concentration in the gas stream increases, the costs associated with replacement of the absorption fluids increases significantly.
Another process, involving adsorption systems, has been used to remove carbon dioxide from gas streams. However, besides being burdened with substantial irreversible energy losses, such systems are also generally limited to the removal of small quantities of carbon dioxide from gas streams because of economic considerations.
Yet another process employs cryogenic processing techniques. At low carbon dioxide concentrations, advantage is taken of the relatively high freezing point of carbon dioxide relative to the freezing point of other gases with which it is normally found in admixtures, by allowing carbon dioxide to selectively freeze out or plate onto heat transfer surfaces; with the subsequent removal therefrom by flowing an essentially carbon dioxide-free gas stream thereover on a subsequent cycle. Unfortunately, at high carbon dioxide concentrations, the freezing carbon dioxide may plug process piping and equipment, rendering the entire system inoperable.
Accordingly, the present invention is directed toward a new technique for economically separating the carbon dioxide from valuable hydrocarbons and then reinjecting the carbon dioxide into the subterranean formation, which avoids the above noted problems of the art.
Applicant is not aware of any prior art references which, in his judgment as one skilled in the carbon dioxide flooding art, would anticipate or render obvious the novel process of the instant invention; however, for the purposes of fully developing the background of the invention and establishing the state of the requisite art, the following are set forth: U.S. Pat. Nos. 4,441,900; 3,995,693; 4,187,910; 3,100,697; 3,116,136; 3,330,124; 4,449,994; 4,318,723; 3,360,945; 3,292,381; 4,417,449.